2006 Saturn Ion stalled

I'm thinking I have an electrical problem with my car and my dealership can't find anything. I got the car in April of '06 and in January the radio/onstar and blinkers all at once went funny. I cranked up the radio and heard nothing, I hit the ONstar button and saw the little phone come on my screen but again heard nothing. When I hit the blinkers they ticked really fast. I turned the car off and back on and everything went back to normal. I reported it to my dealership immediately but they couldn't find anything. It's been happening off and on since but they told me not to worry about it. Yesterday, I actually picked it up from there (they were only doing body work to the top of my trunk from a shingle that hit it) and less than half a mile up the street at an intersection my car sputtered, all the dash lights came on, I lost my abs and couldn't stop the car. I couldn't even turn the car off or pull the key. I eventually got help and the car back to the dealership and they are telling me they have no record of any problems and nothing is wrong and if it happens again it's just going to happen. I do a lot of highway driving and I'm afraid that if it happens again it's going to be too late. Do you have any ideas as to what it could be? I'm seriously afraid to go pick it up today. Thank you for advice.